The nephridium (pl.: nephridia) is an invertebrate organ, found in pairs and performing a function similar to the vertebrate kidneys (which originated from the chordate nephridia). Nephridia remove metabolic wastes from an animal's body. Nephridia come in two basic categories: metanephridia and protonephridia. All nephridia- and kidney- having animals belong to the cladeNephrozoa.
A metanephridium typically consists of the nephrostome (a ciliated funnel) opening into the body cavity, connected to a duct which may be variously glandularized, folded or expanded (vesiculate) and which typically opens to the organism's exterior. These ciliated tubules pump water carrying surplus ions, metabolic waste, toxins from food, and useless hormones out of the organism by directing them down funnel-shaped bodies called nephrostomes. This waste is passed out of the organism at the nephridiopore. The primary urine produced by filtration of blood (or a similarly functioning fluid) is modified into secondary urine through selective reabsorption by the cells lining the metanephridium.
Saccate metanephridia
The saccate metanephridia are excretory glands which function similarly to the metanephridia. They are found in the arthropods: coxal glands of arachnids, antennal (or green) glands and maxillary glands of crustaceans, etc.
The saccate metanephridia filter the fluid of the hemocoel, as opposed to the metanephridia which filter coelomic fluid. In a saccate metanephridium, there is a ciliated funnel covered with a membrane that helps to filter the hemocoel of heavy particles (such as proteins and carbohydrates) before the fluid even enters the funnel. Inside the funnel, the fluid is further processed through selective reabsorption, and eventually excreted from the nephridiopore.
In Crustacea, the saccate metanephridia are associated with the antennae and form the antennal gland. In freshwater crustacea, the saccate metanephridia are especially large due to their role in osmoregulation; crustacea must remove large amounts of water from the tissues, as the cells are hypertonic to the surrounding water.
Protonephridia
Flatworm flame cell
A protonephridium (proto = "first") is found in the phyla Platyhelminthes, Nemertea, Rotifera and Chordata (lancelets). They have the same anatomy as the metanephridia but with the internal ciliated funnel blocked by terminal cells: either a flame cells (if ciliated) or a solenocytes (if flagellated). Thus their tubules lack internal openings, while retaining their opening to the organism's exterior. They function in osmoregulation (ionoregulation).
Each terminal cell has one or more cilia and their beating inside the protonephridial tube creates an outward going current and hence a partial pressurization in the blind of the tube. Because of this, pressurization drives waste fluids from the inside of the animal, and they are pulled through small perforations in the terminal cells and into the protonephridium. The perforations in the terminal cell are large enough for small molecules to pass, but larger proteins are retained within the animal. From the bottom of the protonephridium the solutes are led through the tube, formed by the canal cells, and exits the animal from a small opening formed by the nephridiopore.
Selective reabsorption of useful molecules by the canal cells occurs as the solutes pass down the tubule. Protonephridia are generally found in basal organisms such as flatworms. Protonephridia likely first arose as a way to cope with a hypotonic environment by removing excess water from the organism (osmoregulation). Their use as excretory and ionoregulatory structures likely arose secondarily.
These are excretory systems in phyla Platyhelminthes and are also called blind tubules. These tubules bear a tuft of cilia or flagellum. An organ of excretion in flatworms: a hollow cup-shaped cell containing a bunch of cilia or flagellum, whose movement draws in waste products and wafts them to the outside through a connecting tubule.
Baeumler N., Haszprunar G. & Ruthensteiner B. (2012). "Development of the excretory system in a polyplacophoran mollusc: stages in metanephridial system development". Frontiers in Zoology9: 23. doi:10.1186/1742-9994-9-23.
The gastrotrichs, commonly referred to as hairybellies or hairybacks, are a group of microscopic (0.06–3.0 mm), cylindrical, acoelomate animals, and are widely distributed and abundant in freshwater and marine environments. They are mostly benthic and live within the periphyton, the layer of tiny organisms and detritus that is found on the seabed and the beds of other water bodies. The majority live on and between particles of sediment or on other submerged surfaces, but a few species are terrestrial and live on land in the film of water surrounding grains of soil. Gastrotrichs are divided into two orders, the Macrodasyida which are marine, and the Chaetonotida, some of which are marine and some freshwater. Nearly 800 species of gastrotrich have been described.
Nemertea is a phylum of animals also known as ribbon worms or proboscis worms, consisting of about 1300 known species. Most ribbon worms are very slim, usually only a few millimeters wide, although a few have relatively short but wide bodies. Many have patterns of yellow, orange, red and green coloration. The foregut, stomach and intestine run a little below the midline of the body, the anus is at the tip of the tail, and the mouth is under the front. A little above the gut is the rhynchocoel, a cavity which mostly runs above the midline and ends a little short of the rear of the body. All species have a proboscis which lies in the rhynchocoel when inactive but everts to emerge just above the mouth to capture the animal's prey with venom. A highly extensible muscle in the back of the rhynchocoel pulls the proboscis in when an attack ends. A few species with stubby bodies filter feed and have suckers at the front and back ends, with which they attach to a host.
The excretory system is a passive biological system that removes excess, unnecessary materials from the body fluids of an organism, so as to help maintain internal chemical homeostasis and prevent damage to the body. The dual function of excretory systems is the elimination of the waste products of metabolism and to drain the body of used up and broken down components in a liquid and gaseous state. In humans and other amniotes, most of these substances leave the body as urine and to some degree exhalation, mammals also expel them through sweating.
The nephron is the minute or microscopic structural and functional unit of the kidney. It is composed of a renal corpuscle and a renal tubule. The renal corpuscle consists of a tuft of capillaries called a glomerulus and a cup-shaped structure called Bowman's capsule. The renal tubule extends from the capsule. The capsule and tubule are connected and are composed of epithelial cells with a lumen. A healthy adult has 1 to 1.5 million nephrons in each kidney. Blood is filtered as it passes through three layers: the endothelial cells of the capillary wall, its basement membrane, and between the podocyte foot processes of the lining of the capsule. The tubule has adjacent peritubular capillaries that run between the descending and ascending portions of the tubule. As the fluid from the capsule flows down into the tubule, it is processed by the epithelial cells lining the tubule: water is reabsorbed and substances are exchanged ; first with the interstitial fluid outside the tubules, and then into the plasma in the adjacent peritubular capillaries through the endothelial cells lining that capillary. This process regulates the volume of body fluid as well as levels of many body substances. At the end of the tubule, the remaining fluid—urine—exits: it is composed of water, metabolic waste, and toxins.
Excretion is elimination of metabolic waste, which is an essential process in all organisms. In vertebrates, this is primarily carried out by the lungs, kidneys, and skin. This is in contrast with secretion, where the substance may have specific tasks after leaving the cell. For example, placental mammals expel urine from the bladder through the urethra, which is part of the excretory system. Unicellular organisms discharge waste products directly through the surface of the cell.
Renal physiology is the study of the physiology of the kidney. This encompasses all functions of the kidney, including maintenance of acid-base balance; regulation of fluid balance; regulation of sodium, potassium, and other electrolytes; clearance of toxins; absorption of glucose, amino acids, and other small molecules; regulation of blood pressure; production of various hormones, such as erythropoietin; and activation of vitamin D.
The Turbellaria are one of the traditional sub-divisions of the phylum Platyhelminthes (flatworms), and include all the sub-groups that are not exclusively parasitic. There are about 4,500 species, which range from 1 mm (0.039 in) to large freshwater forms more than 500 mm (20 in) long or terrestrial species like Bipalium kewense which can reach 600 mm (24 in) in length. All the larger forms are flat with ribbon-like or leaf-like shapes, since their lack of respiratory and circulatory systems means that they have to rely on diffusion for internal transport of metabolites. However, many of the smaller forms are round in cross section. Most are predators, and all live in water or in moist terrestrial environments. Most forms reproduce sexually and with few exceptions all are simultaneous hermaphrodites.
The Malpighian tubule system is a type of excretory and osmoregulatory system found in some insects, myriapods, arachnids and tardigrades.
A flame cell is a specialized excretory cell found in simple invertebrates, including flatworms (Platyhelminthes), rotifers and nemerteans; these are the simplest animals to have a dedicated excretory system. Flame cells function like a kidney, removing waste materials. Bundles of flame cells are called protonephridia.
A nephridiopore is part of the nephridium, an excretory organ found in many organisms, such as flatworms and annelids. Polychaetes typically release their gametes into the water column using nephridiopores.
The coxal gland is a gland found in some arthropods, for collecting and excreting urine. They are found in all arachnids, and in other chelicerates, such as horseshoe crabs. The coxal gland is thought to be homologous with the antennal gland of crustaceans. The gland consists of an end sac (saccule), a long duct (labyrinth) and a terminal bladder (reservoir). There is generally only one pair, and they open on the coxae of the walking legs. The coxal secretion of adult female ticks of Ornithodoros erraticus contains a sex pheromone.
Pronephros is the most basic of the three excretory organs that develop in vertebrates, corresponding to the first stage of kidney development. It is succeeded by the mesonephros, which in fish and amphibians remains as the adult kidney. In amniotes, the mesonephros is the embryonic kidney and a more complex metanephros acts as the adult kidney. Once a more advanced kidney forms, the previous version typically degenerates by apoptosis or becomes part of the male reproductive system.
The epithelial sodium channel(ENaC), (also known as amiloride-sensitive sodium channel) is a membrane-bound ion channel that is selectively permeable to sodium ions (Na+). It is assembled as a heterotrimer composed of three homologous subunits α or δ, β, and γ, These subunits are encoded by four genes: SCNN1A, SCNN1B, SCNN1G, and SCNN1D. The ENaC is involved primarily in the reabsorption of sodium ions at the collecting ducts of the kidney's nephrons. In addition to being implicated in diseases where fluid balance across epithelial membranes is perturbed, including pulmonary edema, cystic fibrosis, COPD and COVID-19, proteolyzed forms of ENaC function as the human salt taste receptor.
The salt gland is an organ for excreting excess salts. It is found in the cartilaginous fishes subclass elasmobranchii, seabirds, and some reptiles. Salt glands can be found in the rectum of sharks. Birds and reptiles have salt glands located in or on the skull, usually in the eyes, nose, or mouth. These glands are lobed containing many secretory tubules which radiate outward from the excretory canal at the center. Secretory tubules are lined with a single layer of epithelial cells. The diameter and length of these glands vary depending on the salt uptake of the species.
Osmoregulation is the active regulation of the osmotic pressure of an organism's body fluids, detected by osmoreceptors, to maintain the homeostasis of the organism's water content; that is, it maintains the fluid balance and the concentration of electrolytes to keep the body fluids from becoming too diluted or concentrated. Osmotic pressure is a measure of the tendency of water to move into one solution from another by osmosis. The higher the osmotic pressure of a solution, the more water tends to move into it. Pressure must be exerted on the hypertonic side of a selectively permeable membrane to prevent diffusion of water by osmosis from the side containing pure water.
Phoronids are a small phylum of marine animals that filter-feed with a lophophore, and build upright tubes of chitin to support and protect their soft bodies. They live in most of the oceans and seas, including the Arctic Ocean but excluding the Antarctic Ocean, and between the intertidal zone and about 400 meters down. Most adult phoronids are 2 cm long and about 1.5 mm wide, although the largest are 50 cm long.
The excretory system of gastropods removes nitrogenous waste and maintains the internal water balance of these creatures, commonly referred to as snails and slugs. The primary organ of excretion is a nephridium.
In biology, solenocytes are elongated, flagellated cells commonly found in lower invertebrates, such as flatworms, chordates and several other animal species. In terms of function, solenocytes play a significant role in the excretory systems of their host organism(s). For example, the lancelets, also referred to as amphioxus, utilize solenocytic protonephridia to perform excretion. In addition to excretion, these cells contribute to ion regulation and osmoregulation. With this in mind, solenocytes form subtypes of protonephridium and are often compared to another specialized excretory cell type, i.e., flame cells. Solenocytes have flagella, while flame cells are generally ciliated.
The rock dove, Columbia livia, has a number of special adaptations for regulating water uptake and loss.
The kidneys are a pair of organs of the excretory system in vertebrates, which maintain the balance of water and electrolytes in the body (osmoregulation), filter the blood, remove metabolic waste products, and, in many vertebrates, also produce hormones and maintain blood pressure. In healthy vertebrates, the kidneys maintain homeostasis of extracellular fluid in the body. When the blood is being filtered, the kidneys form urine, which consists of water and excess or unnecessary substances, the urine is then excreted from the body through other organs, which in vertebrates, depending on the species, may include the ureter, urinary bladder, cloaca, and urethra.
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